In order to meet the thrust requirements of military applications, military turbo-fan engines employ afterburning. During after-burning, the engine adds and ignites fuel in the exhaust of the turbo-fan engine to create additional thrust. A consequence of after-burning is that when the exhaust burns it becomes less dense and requires opening the nozzle of the engine to maintain air flow rate. Insufficient nozzle area creates back-pressure on the fan. This back pressure can cause the engine to stall.
A prior approach to prevent stalling during after-burning mode of a turbo-fan engine is to increase the cross-sectional flow area of the exhaust nozzle of the engine during after-burning. This allows the exhaust that the engine burns to escape more easily from the engine, thereby relieving the back pressure. The opening and closing of the engine's nozzle has the appearance of the iris of the human eye as the eye reacts to light.
Using an engine with an adjustable exhaust nozzle has many inherent disadvantages which penalize aircraft performance. The disadvantages include the mechanical complexity of the engine, the high weight of the engine's nozzle, the high cost of the nozzle, poor reliability and maintainability of the nozzle, adverse aerodynamic integration of the nozzle into the air frame, poor radar and thermal observables due to edges and gaps, difficult radar and thermal observables cooling and treatments, leakage losses, and difficult structural integration of the engine into the air frame.
In some military applications, the disadvantages associated with a variable geometry nozzle have prevented the use of an engine with after-burning capability. This is particularly important when trying to achieve low radar and thermal observables for stealth or radar avoiding aircraft. Therefore, currently available aircraft with stealth properties, such as the B-2 bomber, F-117 fighter, and A-12 attack aircraft all have fixed geometry nozzles, at the expense of no after-burning capability. The lack of after-burning capability denies these aircrafts the additional thrust available through after-burning, and therefore makes the plane and its crew more vulnerable to destruction.
A second approach to eliminate the inherent disadvantages of a variable geometry exhaust nozzle has been to provide an after-burning engine with an exhaust nozzle with a fixed aperture at the exit, but still having the capability and need to vary the internal flow area of the exhaust nozzle during after-burner mode. An inherent problem with this approach is that the variation of the nozzle throat, while maintaining the exit aperture fixed, produces a non-optimum area ratio in the nozzle which results in loss of thrust from the engine.